1. Field of the Invention
The present invention relates to a method of controlling annual weeds in a flooded field by effective application of a selective post-emergence herbicide of 3,4-dichloropropionanilide in a flooded condition.
2. Description of the Prior Art
Heretofore, it has been known that 3,4-dichloropropionanilide (DCPA) is a herbicide for foliage treatment which has no phytotoxicity to rice plants and is effective in controlling gramineous weeds, especially barnyard grass, and also young broadleaf weeds. Accordingly, DCPA has been widely used as a herbicide in paddy fields. However, it is also well-known that the herbicidal activity of DCPA is decreased in the presence of water, whereby desirable herbicidal effects cannot be achieved when the fields are in a flooded condition. Furthermore, in order to effectively apply DCPA, the surface of the field should be exposed before application and the soil surface should be maintained in an unwatered state for 2 to 3 days. This requirement of water management before and after treatment and the attendant control of the water supply which is required cause inconveniences in the field application of DCPA. Accordingly, DCPA has been applied to rice growing areas in upland fields, in upland nursery beds, in paddy fields after the water has been drained and in directly seeded paddy fields after the water has been drained. In the direct-seeding culturing of paddy rice under water-drained field conditions, the field is later flooded. Accordingly, 97% of the world's rice has been cultured in paddy fields, of which 30 - 40% are ill-drained, i.e., where surface drainage cannot be attained. Consequently, notwithstanding the outstanding phytophysiological selectivity of DCPA to rice, the contribution of DCPA to the world's rice cropping industry is small.
In Japan, transplanting culturing has been used in most fields. Accordingly, the application of DCPA has been quite limited. However, there is no other foliage-treating type herbicide which has selectivity to barnyard grass without phytotoxicity to rice plants. Accordingly, several studies have been undertaken to develop techniques for applying DCPA to fields in flooded conditions, as disclosed in Japanese Patent Publications No. 2080/1967, No. 1490/1968 and No. 16934/1969. In these cases, DCPA is used as the main component and certain auxiliary ingredients are added so as to impart a herbicidal activity in the presence of water. Japanese Patent Publication No. 2080/1967 discloses the combination of DCPA and one of certain phenyl or naphthyl carbamates, thiophosphates or trialkyl tin compounds. By using the proper proportions, the composition is used as a contact herbicide for foliage treatment of paddy fields in a flooded condition. This method has been considered to be effective. However, the phytophysiological selectivity toward barnyard grass versus rice plant is remarkably decreased by the addition of the second ingredient as compared with the application of DCPA alone. Accordingly, the field and rice plant conditions under which the composition is effective are quite limited. This is a significant disadvantage in practical application. The conclusion has been reinforced in several studies of the method's efficacy. The method of Japanese Patent Publication No. 2080/1967 also has other disadvantages. It has been found that phytotoxicity is caused by DCPA when certain commercial carbamate or organic phosphorus insecticides are applied within 10 days of the application of DCPA. This is a serious disadvantage for use of DCPA. Using the foliage treating method, it has been difficult to overcome these disadvantages. Accordingly, the invention of Japanese Patent Publication No. 2080/1967 has not been used in practice.
As the result of a series of studies, the composition of Japanese Patent Publication No. 16934/1974 has been registered and used in practice. The composition comprises DCPA and S-(4-chlorobenzyl)-N,N-diethylcarbamate as active ingredients.
The composition of Japanese Patent Publication No. 2080/1967 comprises DCPA and methyl-N-(3,4-dichlorophenyl)carbamate as active ingredients. In both cases, it has been necessary to apply more than 50 g per 1 are (100 m.sup.2) to obtain satisfactory results.
A characteristic feature of all of these Japanese patent publications is the determination of suitable methods of application of DCPA. That is, it has been considered in the past that the herbicidal effect cannot be attained without spraying the composition of DCPA onto the stems and leaves of the plant (foliage treatment). In accordance with studies of methods of application of DCPA to plants in a flooded field, it has been found that the selective herbicidal effect of DCPA can be attained by dispersing certain selected second components in the flooding water. In accordance with this method, the herbicidal components are not directly contacted with the rice plants above the water surface. Accordingly, the phytotoxicity caused by subsequently applying an insecticide in the presence of DCPA can be remarkably decreased. However, these methods also have disadvantages. Firstly, the suitable leaf stage for controlling barnyard grass is 2 - 3 leaf stage or lower. However, it is desirable to be able to control barnyard grass in higher stages of leaf growth because of attendant labor savings. Secondly, in order to attain the herbicidal effect in flooded waters, a quite large amount of DCPA has to be applied in said methods. But this requirement is contrary to the latest theories of the technology of concern, and it is necessary to decrease the amount of DCPA to at least that of the foliage treatment.
In summary, then, two methods of application of DCPA have been used: foliage treatment or dispersal in the flooding water. But both have significant disadvantages. Consequently, it would be most desirable to find a second ingredient so as to improve the selectivity of the herbicidal effect on barnyard grass relative to rice plants (as in the selectivity of DCPA itself) and to increase the herbicidal activity of DCPA while overcoming the disadvantages of the present application methods.